1. Field of the Invention
The present invention relates to an electrostatic image developing machine and in particular to a developing apparatus for developing an electrostatic image produced on a recording or an image receiving medium in accordance with electrostatic recording operation by using a developing agent of single-component series which includes no carrier but only toner as a main component.
2. Description of the Prior Art
In general, as the method for developing an electrostatic latent image produced by an electrophotographic or electrostatic recording process, there have been hitherto known a magnetic-brush developing method, a cascade developing method or the like in which a developing composition of two-component series (i.e. containing at least two components, carrier particles and toner particles) is used. Although these methods are certainly advantageous in that an image having excellent image quality can be reproduced, there are common drawbacks ascribable to the two-component developing compositions such that the carrier undergoes fatigue and the mixing ratio of carrier and toner is undesirably subjected to variation.
The disadvantages described above are essentially absent in the case of the developing method in which the developing agent of a single-component series containing no carrier but only the toner as a main component is used.
Lately, there has been developed a developing machine such as shown in FIG. 1 which is adapted to developing the electrostatic latent image by using the developing agent of single-component series containing as a main component a toner which exhibits ferromagnetism. Referring to FIG. 1, the developing apparatus comprises a rotatable drum 1 having a photoelectric light sensitive layer deposited on the outer peripheral surface thereof and a cylindrical rotatable sleeve 3 of a non-magnetic material which is disposed in opposition to the drum 1 with a gap 2 relative to the outer periphery of the latter, the sleeve 3 being adapted to be rotated in the direction opposite to that of the drum 1, as indicated by respective arrows. A tonor container 5 is provided in combination with the non-magnetic rotating sleeve 3 so as to apply the magnetic toner 4 to a circumferential surface of the sleeve 3 at a location exept for the region defining the gap 2. Disposed stationarily within the rotatable sleeve 3, a small distance from the inner peripheral wall thereof, are a developing magnet 6 at a position aligned with the gap 2, a toner introducing magnet 7 at a position upstream of the developing magnet 6 as viewed in the rotating direction of the sleeve 3, a toner feedback magnet 8 positioned downstream of the developing magnet 6, and a series of toner feeding or transporting magnets 9, 10 and 11 for feeding the toner towards the gap 2 from the toner container 5. These magnets 6 to 11 are fixedly supported by a holding member 12 and so arrayed that the polarities of these magnets are alternately reversed as viewed in the rotating direction of the sleeve 3 and as shown in FIG. 1. Reference numeral 13 denotes a trimming plate for regulating the thickness of toner layer (not identified by a reference numerical).
With the structure of the developing machine described above, the toner 4 is deposited on the outer peripheral surface of the cylindrical sleeve 3 under the influence of the magnetic force, for example magnets 9 and 10, and fed out from the toner container 5 through the trimming plate 13, in the form of a toner layer having a constant thickness, as the sleeve 3 is rotated. Then the toner layer is successively moved to gap 2 according to the rotation of the sleeve 3 under the magnetic force of magnets 11, 7 and 6. At the region of the gap 2, the toner layer is caused to fluff or expand radially outwardly under the action of a magnetic field produced by the developing magnet 6 in cooperation with the toner introducing magnet 7 and the toner feedback magnet 8, whereby the toner is brought into sliding contact with the outer peripheral surface of the rotating drum 1. Accordingly, by imparting a required quantity of electric charge to the toner, the electrostatic latent image produced in the light sensitive layer 14 (see FIG. 2) of the rotating drum 1 will attract the toner particles which are thus deposited on the outer peripheral surface of the drum in a pattern corresponding to the latent image, thereby to develop the electrostatic latent image. The toner image thus developed is subsequently transferred to a conventional copy sheet such as plain paper and fixed through a well known process.
The developing apparatus described above suffers from drawbacks ascribable to the use of the magnetic toner. More specifically, the toner particles will likely tend to be deposited in the form of particle stacks on the surface of the light sensitive layer 14 of the drum 1 which serves as the image carrier, as is illustrated in FIG. 2, in which the toner particle stacks are denoted by reference numeral 15. Such particles stacks will involve degradation in the sharpness of the toner image itself and hence result in remarkable deterioration in the quality of the reproduced image after the transfer to the copy sheet, because the toner particle stacks 15 are disintegrated during the transfer process which is carried out with the copy sheet being snugly pressed against the light sensitive layer 14 of the drum 1, whereby the loose toner particles will be dispersed outwardly beyond the outline of the image to be reproduced, thereby to blur the image.
In addition, it is observed that some toner particles will tend to remain as deposited on the light sensitive layer 14 in the peripheral area around an inherent image region 16, as the result of which a shadow image 17 is developed to blur the reproduced image, as illustrated in FIG. 3.
Although the causality of the phenomena described above has not been completely elucidated yet, it is believed that the occurrence of such undesirable phenomena is ascribable to the absence of the carrier exhibiting a scraping action. In conjunction with this, it should be mentioned that the carrier is usually present in a quantity about one hundred times as large as the toner.
Further, the developing apparatus shown in FIG. 1 has an additional disadvantage in that the height of the bulge or rise-up of the toner particles produced under the influence of the developing magnet 6 and so forth will remain relatively low because the developing agent as employed is of the single-component series, whereby the gap 2 can not be implemented in a large size, imposing a high precision requirement in respect of the configurations and positional relationship of the rotating drum 1 and sleeve 3.